I know what you mean, I have my flat rented in Girona. Lucky me, I've had only two different couples renting the flat for the last 8 years, but even that they gave me back all the keys when they left the flat, you can't know if they have more copies. It's supposed that you can't copy "modern" keys, but actually, if you go to any "key copy" shop, you won't have any problem to copy them. At least in Catalonia. Incredible but true. So the best thing is before renting a flat be sure that they change the lock, because you are not allowed to do it after signing the contract without owner permission (and they won't be interested in changing it).
Hope all goes well with the new place!